Screening Methods and Sources of Varietal ResistanceSeung Yoon Choi

Entomologist, College of Agriculture, Seoul National University, Suweon, Korea.

The methodology of mass screening rice at the seedling stage and the sources of resistance to the brown planthopper Nilaparvata lugens (Stål) that have been identified are discussed. The insect can be efficiently mass-reared on the susceptible varieties on 40- to 50-day-old potted plants or on seedling mats. Many countries have adopted the basic screening procedures developed by the International Rice Research Institute. A large number of varieties or lines that are sources of genetic resistance to the insect are available in the rice germplasm. However, their resistance in one country does not necessarily hold true in other countries.

THE INTERNATIONAL RICE RESEARCH INSTITUTE (IRRI) initiated studies on varietal resistance to the brown planthopper (BPH) Nilaparvata lugens (Stål) in 1966. Since then, it has developed and documented efficient mass-screening techniques for evaluating tens of thousands of varieties from worldwide germplasm collections in the seedling stage in the greenhouse.

The basic techniques have been introduced in Japan (Kaneda and Kisimoto 1977), Korea (Choi et al 1977), Taiwan (Cheng and Chang 1977), India (Kalode and Krishna 1977), Thailand (Pongprasert and Weerapat 1977), Sri Lanka (Fernando et al. 1977), Indonesia (Harahap 1977), and the Solomon Islands (Stapley et al. 1977). With those techniques, the varieties from the germplasm collections and the bulk of those countries breeding materials have been satisfactorily screened in the seedling stage for resistance to the BPH. This paper reviews the details of mass-screening techniques presently used and the resistant sources identified so far.

SCREENING METHODS

Mass-rearing of the brown planthopperMass-rearing of the BPH is essential to mass screening of varieties. The insect is mass-reared on the susceptible variety Taichung Native 1 or on other equally susceptible varieties that provide food and sites for oviposition. The original colony is started by caging a pair (♀♂) of adults (virus-free insects from areas harboring the grassy stunt virus) on the rice plant.

Two methods of mass-rearing are employed.

Rearing on old potted plants. Pathak and Khush (1977) at IRRI described this method, and most countries use it. The insects are reared on 40- to 50- day-old plants inside a 0.5- × 0.5- × 1-m cage. This standard cage uses a wooden or steel frame, bottom, and door. Its roof and a side wall are covered with glass, but the remaining two sides are covered with either fine-mesh wire screen or nylon cloth. The screen permits aeration and prevents condensation of moisture on the glass walls. The potted plants are changed as needed. Each cage can accommodate several potted plants that usually support 2,000 to 3,000 late-instar nymphs. Eggs of about the same age are obtained by placing the plants overnight in a cage containing adult insects.

Rearing on seedling mats. Insects are mass-reared on seedlings in a transparent acrylic (plastic) cage. The top and three sides of the cage are covered with fine-mesh nylon cloth. Japan (Kaneda and Kisimoto 1977) and Korea (Choi et a1 1977) use this method. Choi et al. (1977) have also described it. Seedling mats are prepared by placing the pregerminated seeds on moistened gauze on enameled trays. The mats can be easily cut into the desired size several days later when the seedlings have grown and their roots have become entangled in the gauze. The adult insects are caged on the seedling mats. The

ovipositing insects are removed 1 or 2 days later and the seedling mats with eggs are taken out of the cages. A fresh seedling mat is put on one side inside the cage as nymphs appear. Each cage can accommodate one seedling mat (about 20 × 25 cm) that can support 1,000 to 2,000 late-instar nymphs.

Insects are reared in this method in insectaries at 26-27°C and with 15-hours light in Japan (Kaneda and Kisimoto 1977) and at 25-30°C and with 24-hours light in Korea (Choi et a1 1977).

Rearing on old potted plants and on seedling mats provide a continuous supply of test insects. The use of rice seedling mats is feasible in insectaries in the off-season in the temperate zone, but their efficiency is inferior to that of potted plants in the greenhouse. In Korea, the former method is used in summer, and the latter, during other seasons.

Screening procedures. In many countries, varietal screening for resistance to the BPH is usually conducted at the seedling stage in the greenhouse. Basic screening procedures standardized by IRRI (Pathak and Khush 1977) have been adopted in different countries.

The test varieties or lines are seeded in rows 5 cm apart in 60- × 45- × 10-cm seed boxes. Each line is planted in a 20-cm-long row along the width of the seed box. A susceptible check variety (usually Taichung Native 1 or one of the equally susceptible varieties) and a resistant check variety (always Mudgo, except in certain biotype studies) are planted at random in each seed box. At 7 days after seeding, the test plants are thinned to 20 to 30 seedlings per row. Initial evaluation uses only one replicate.

Sometimes germination is not uniform because the seeds germinate poorly or are infected with soil-borne disease. The problems can be overcome by pre-germinating the seeds and disinfecting them with fungicides like phenyl mercuric acetate (Choi et al 1977).

The pre-germinated seeds are planted individually at uniform spacing within each row. Although laborious, this method is superior to direct seeding in ensuring a uniform stand of seedlings and eliminating thinning work.

The seeded boxes are placed on a galvanized iron tray on a table inside a screened room. The screened room may be a section of a greenhouse. About 5 cm of standing water on the tray provides high humidity suitable for insect survival and eliminates the need for watering the plants, which may disturb the insects feeding on them.

If a screened room is not available, each seed box may be put in a wooden cage covered with fine-mesh cloth (Choi et al 1977). In Taiwan, the seeded boxes are covered with a nylon net after infestation (Cheng and Chang 1977).

About 7 days after seeding at the one- and the two-leaf stage, the seedlings are infested by scattering a large number of insects on them. The heavily infested plants from the mass-rearing cage are gently tapped over the seedlings. The insects should be on the test varieties as uniformly as possible. Generally second- and third-instar nymphs are used for infestation. An average of 5 insects/seedling constitutes an optimum population to differentiate the resistant and susceptible lines.

The insects' preference or non-preference for the test varieties is recorded. But the final rating for resistance is based on the extent of damage the different test varieties suffer. Up to 1975, a scoring system with a scale of 0 to 5 was used. In 1976, IRRI revised the scoring system (IRTP 1975). At present, plant damage is rated on the standard scoring system of 0-9 (Table 1).

The final damage rating is taken when about 90% of the plants of the susceptible check variety are killed—usually about 7 to 10 days after infestation.

The varieties or lines that fall into grade 1 to 3 (or 0 to 2 under the former scoring system) are further evaluated for consistency of resistance in a repeat screening using the same technique, with four replicates per entry. The selected lines or varieties are tested further to evaluate the nature and causes of their mechanisms of resistance.

SOURCES OF RESISTANCE

Since 1966, rice entomologists and breeders at IRRI have given much attention to the search for sources of resistant genes and the use of resistance in their breeding programs. Since then, screening for BPH resistance has been done on tens of thousands of varieties and lines from IRRI’s worldwide germplasm collections and on its breeding lines. From these screening studies, a large number of varieties originating from indica rice were identified as resistant.

Using the mass-screening technique, many Asian rice-growing countries have also screened their available rice germplasm and breeding lines and incorporated the sources of resistance compatible with other desirable plant characters in their breeding programs.

More than 500 varieties or lines have been found resistant to the BPH (Table 2) in the Philippines (IRRI 1972, 1973, 1974, 1976; IRTP 1975, 1976a, b; Pathak and Khush 1977), Japan (Kaneda and Kisimoto 1977), Korea (Song et al. 1972; Choi 1975; Choi et al. 1977), Taiwan (Cheng and Chang 1977), Thailand (Pongprasert and Weerapat 1977), Indonesia (Harahap 1977), India (Kalode and Krishna 1977), Sri Lanka (Fernando et al. 1977), and the Solomon Islands (Stapley et al. 1977). Varieties resistant in a country were not, however, necessarily resistant in other countries. For example, the varieties ASD7 and Mudgo are resistant in the Philippines (IRRI), Japan, Korea, Taiwan, Thailand, and Indonesia, but susceptible in India.

The different reactions could be due to the presence of different populations of BPH in different locations and countries. In the future, the sources of resistance so far identified should be retested against the different biotypes to tackle the development of biotypes capable of surviving on the resistant plants.

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-----Citation:Seung Yoon Choi. 1979. Screening methods and sources of varietal resistance. Pages 171-186. In: Brown Planthopper: Threat to Rice Production in Asia. International Rice Research Institute, Los Baños, Philippines, 369 pages.